In control plane managed networks (both distributed e.g. GMPLS and centralized e.g. SDN) it is not possible to ensure return to the original (nominal) path of traffic restored on a restoration path in response to a failure affecting the nominal path after the failure is repaired.
Existing control plane technology is based on two recovery mechanisms: protection and restoration. In the case of protection two paths are always provided, called working and protection paths. The two paths are always committed (i.e. resources dedicated to a given circuit and hence traffic flow) and depending on failures traffic is moved from worker to protection and vice-versa. In the case of restoration, a single path at time is dedicated to a given circuit. When a failure occurs a new path is computed and all the traffic is moved to the restoration path.
When a network is planned, the nominal path between two nodes is computed and provided. This nominal path is computed in a way taking into consideration requirements of the traffic and what the network has to offer. From this point of view the nominal path is an optimal path for the traffic.
A communications network is often large and complex structure and failures affecting traffic in the network happen from time to time. These could be software or hardware failures in network nodes or physical failures affecting links between nodes. For example an optical fibre link may be cut during ground works. It may also happen that a number of failures occur in a short period of time. Because the network operator has contracts for delivering data maintaining connections between nodes is essential for sustainability of the business. In consequence a network fault triggers protection and/or restoration. However, in the case of restoration, after a number of failures topology of the network is changed and the paths linking nodes no longer use the same resources. In consequence when a fault is repaired the traffic that has been restored on a restoration path cannot be returned to the nominal path because the resources of the nominal path have been used for traffic restoration of other paths affected by other faults. This situation is shown in FIG. 13. When a number of network faults occurred the network might fall in a condition when moving the circuits back to their optimal paths is not possible or would require extremely difficult and traffic affecting operations.
In consequence network faults (especially multiple faults) make it impossible to return to nominal path(s), which, in turn, leads to operating the network based on sub-optimal routing.